It should cost around $30(give or take $10, depending on location) a month to run those two lights for 18 hours a day, which won't be noticeable to the electric company. I'd also recommend that you use the reflectors that come with the lights, or alot of their output will be lost. The only reason I would stop using them, is if they happen to have extreme 'hotspots'(as mentioned above) and cause burning areas in parts of your garden. They only way to find out is to try it and keep an eye on them. I don't think it'll be a problem, unless they were 600 or 1000 watt lights.